As the skilled ones in the art well know, the fluid circulation pumps assembled in heating systems or plants are commonly referred to as “circulators”.
Current circulators are almost completely supplied with asynchronous motors. Circulators realized with a synchronous motor with a permanent-magnet rotor have become commercially successful only recently.
The most popular and cost-efficient synchronous motors comprise only two stator pole shoes, but, in an effort to overcome vibration problems, fourpole shoes synchronous motors have been used in recent circulators.
For example, a fourpole shoes stator for a synchronous motor of a circulator is disclosed in U.S. Pat. No. 6,452,303 to the same Applicant.
It is also known that the structural scheme of permanent-magnet rotor circulators includes, additionally to a stator having electromagnet with core of laminations and associated coils, also a rotor centrally located among the stator pole shoes.
A shaft extends axially through the rotor and is rotatively supported by bearingmeans, one of its ends being coupled to the operating member of the circulator, specifically a pump impeller.
Furthermore, the rotor is mounted within a tightly sealed housing, e.g. inside a steel tube, which is isolated from the stator, and the motor shaft is rotatively supported by bearings, respectively provided at the tube bottom and near the impeller.
A circulator structure of the above type is disclosed, for instance, in U.S. Pat. No. 6,365,998. The circulator of that patent is quite expensive, yet not entirely free of locking problems due to lime-scale or to impurities contained in the working fluid and revealing especially after long idle periods.
In fact, when the circulator is idle for a long period, usuallyin the summertime, its rotor can be locked at the starting step, this because of lime-scale stocked within the thin gap between the rotor and the retaining tube. The problem may affect the bearings supporting the motor shaft as well.
To set the circulator back to work, it has been common practice to apply a screwdriver slot realized on the motor shaft end opposite to the impeller. By using a screwdriver in this slot it is possible to apply a twisting couple able to unlock the rotor and restore the circulator functioning. Normally, anyway, the circulator rear part, where the shaft end provided with the slot can be reached, is protected by a box-like cover in which the electric and/or electronic driving circuits and components of the motor are arranged.
The protective cover must therefore allow the unlocking slot to be reached, yet keeping the hydraulic seal with respect to the electric parts.
The problem is more serious where the tube housing the rotor is made of a synthetic-plastic material and the rotor bearings must be attached to that tube.
Close rotor-to-tube assembly tolerances require precise alignment of the rotor bearings to prevent vibration problems shortening the circulator operating life.
A possible approach to the problem would be mounting the bearings on elastic, pliant blocks that allow the rotor to be easily self-aligned without compromising its functioning. This is an expensive solution, however, and it involves fairly precise motor assembling procedures.
The aim of the present invention is that of providing a permanent-magnet type of synchronous electric motor, particularly for circulation pumps of heating systems, which motor having appropriate constructional and functional features to allow, in a very simple overall embodiment, one of the rotor bearings to perform the dual function of allowing the rotor self-aligning even under severe operating conditions and allowing the slot in the end of the motor shaft easy to reach in the event of a lock of the pump.